Dye sensitization of zinc oxide photoconductors



United States Patent 3,403,023 DYE SENSITIZATION 0F ZINC OXIDEPHOTOCONDUCTORS Hugh R. Carrington, Binghamton, and Frank M. Gaesser,

Jr., Endicott, N.Y., assignors to GAF Corporation, a

corporation of Delaware No Drawing. Filed May 14, 1965, Ser. No. 455,97112 Claims. (Cl. 961.7)

ABSTRACT OF THE DISCLOSURE Sensitization of photoconductive zinc oxideby the utilization of dye blends having dye components capable ofabsorbing for each of the primary colors, and including componentsdesigned to overcome the tendency of the background to yellow.

The present invention pertains to zinc oxide photoconductive materialsfor use in electrophotographic printing and more particularly to suchmaterials which are panchromatically dye sensitized zinc oxide-bindercoatings having the desi ed speed, balance, background and storagestability.

The commonly practiced form of electrophotographic printing uses as aphotosensitive element a thin zinc oxide photoconductive insulatinglayer coated on a suitably conductive base sheet.

In this 'form of electrophotography, it is customary to deposit: anelectrostatic charge on the surface of the zinc oxide insulating layerwhile the backing layer is either connected to ground or covered with anapproximately equivalent charge of opposite polarity.

The zinc oxide insulating layer is then illuminated by a light andshadow pattern which represents the image to be reproduced. Whereilluminated the hitherto insulating photoconductive layer becomeselectrically semi-conductive and therefore any previously depositedelectrostatic surface charge is dissipated conductively in consonancewith the actinic radiation which each element of area receives. In thisway an electrostatic latent image is formed, the greatest surface chargedensity remaining in unexposed areas and the least in maximallyilluminated portions of the photoconductive element.

The latent image is then rendered visible by development withelectrostatically charged pigment particles or toners which may beapplied to the image as powder clouds, as suspensions in insulatingliquids of low di electric constant or the like. The toner adheresdifferentially to charged and discharged areas thereby rendering themvisibly different. The visible image may be fixed to the backing memberby fusion or may be transferred to another sheet to which it is fused.

Zinc oxide is almost universally used as the photoconductor in suchelectrophotographic printing processes, particularly where the powderimage is fused to the base of the photoconductive recording element.Zinc oxide, however, has one particular property which mitigates againstits most efiicient use for the intended purpose and that is itsrestricted or limited spectral response which is confined essentially tothe ultraviolet. Because of such limitation, best results are onlysecured where U.V. radiation is available. Use of white light containingbut a minimum amount of U.V. very materially reduces the speed ofresponse of the photoconductor during exposure.

This defect has been recognized and efforts have been made to cope withit by dye sensitizing the zinc oxide to make it responsive to light raysin the visible spectrum. This is accomplished by the employment of anorganic dye or dye mixture incorporated with the zinc oxide and havingthe ability of absorbing radiant energy in the ice visible spectrum andtransferring the absorbed energy to the photoconductor. Various dyes anddye blends designed to accomplish this end are mentioned in Greig,U.S.P. 3,052,540 dated Sept. 4, 1962, and Sugarman et al., U.S.P.3,051,569 dated Aug. 28, 1962.

It is represented in the prior art that the dye blends employed will notonly sensitize in the visible spectrum but will give when mixed withzinc oxide a mixture which when viewed by reflected light will have awhite or offwhite appearance. It is also represented that the dye blendswill have several peaks in different regions of the visible spectrum.

It goes without question that the contributions made by the prior arthave improved the response of the zinc oxide to visible radiation andhave expanded employment of the zinc oxide insulating materials topermit more effective image discharge by visible radiation.Unfortunately, however, the improvements have not been completelysatisfactory in meeting the prerequisites of an electrophotographic zincoxide binder coating as regards speed, balance and background andstorage stability.

It would appear self-evident that the ultimate in dye blends forpanchromatic sensitization of the zinc oxide should be one in whichthere is present a dye for each primary color and which absorbs for onlyone primary color, i.e., a blend of a pure cyan, magenta and yellow.Such a dye blend when mixed with zinc oxide and the mixture viewed byreflected radiation would appear as white. However as is well known,such ideal dyes are the most difficult things to come by. Thus there arefew dyes of the desired color or shade which do not have overlappingabsorptions in the visible spectrum. Typically most red dyes have a blueor yellow cast and absorb not only green but red and blue as well; againgreen dyes usually have a yellow cast thus causing absorption of red andblue. Since economy of operation dictates for the most part theutilization of dyes available on the open market, the selection of a dyeblend having optimum characteristics forces the operator to Work withinvery narrow confines.

Rather peculiarly too, the lack of balance found in most zinc oxide dyesensitized systems is traceable to the cyan or green-blue dye. Mostoften such dye oversensitizes in the red region so that on light imageexposure the red record is discharged or burned out excessively. This inturn means that in the recorded image there is a deficiency in the redresponse. It likewise leads to a sensitized element whose storagestability is more than a little suspect.

The problem of proper dye sensitization is further aggravated by thefact that most electrophotographic papers have a decided tendency toyellow and to produce discolored backgrounds. This deleterious effectcan be overcome by use of a masking dye for yellow, i.e., blue. However,in the elimination of this headache, care must be taken not to create anever greater one by the use of a dye which throws the sensitizing dyeblend completely out of color balance.

We have now discovered that we can produce zinc oxide insulatingelements for electrophtographic recording which have the followingcharacteristics:

(1) Proper response to visible light,

(2) High visible light speed,

(3) Storage stability, and

(4) Masked yellow background.

These ends are attained by the utilization of special dye blends havingdye components capable of absorbing for each of the primary colors whileyielding a mixture with zinc oxide which when viewed by reflected lightappears white to gray, said blend including components designed toovercome the tendency of the background to yellow.

The recording element through which we realize our objectives comprisesabacking member which may be rendered conductive, as in conventionalpractice, coated with an insulating binder-resin having a white zincoxide photoconductor dispersed therein. The zinc oxide is treated with adye blend to make it absorptive to light rays of the visible spectrum toensurethat it will have proper speed and balance when irradiatedimagewise with white light. Agents are also present or provide anuncolored background.

The backing member may be any suitable paper base rendered conductive(i.e., 10 ohm cm.) by any of several means.

The zinc oxide is French process zinc oxide such as that sold by the NewJerseyZinc Sales Company Incorporated of New York under the trade namePhotox 801.

The resin binder may be any suitable hydrophobic insulating resin of thetype used in the manufacture of electrophotographic materials. Examplesof such resins are silicone, polystyrene, polyvinyl acetate and thelike. We obtain particularly good results with epoxy ester resins inwhich the ester group is supplied by a fatty acid, i.e., stearic, or arosin acid and abietic acid. Particularly suitable is the modified epoxyresin which is commercially available as Epidene E 168/50 from the T. F.Washburn Co., Chicago, Ill. This compound can be described as an epoxyester polymer derived from bisphenol A, thedi-(4-hydroxybenzene)-dimethylmethane, which is further modified by afatty acid, preferably oleic acid. Epoxy polymers modified by abieticacid or stearic acid are equally suitable. Fatty acid mixturescontaining oleic and linoleic acid and derived from tall oil areespecially preferred.

The contemplated recording elements may be prepared by mixing a solutionof an epoxy ester resin in toluene or xylene with small quantities ofmetallic driers, such as the naphthenates of cobalt and maganese.Typical pigment resin ratios range from 5.5 to 6.0.

- There is also added a plasticizer with the resin. For this purpose weprefer to use a petroleum-derived polymeric hydrocarbon with a fairdegree of aromaticity and relatively low iodine value. A suitableplasticizer meeting these prerequisites is the product sold as VelsicolTPO #1 by Velsicol Chemical Corp., 535 5th Ave., New York, NY.

This product is a resinous, dark polymeric material of low molecularweight derived from a petroleum hydrocarbon and prepared from a mixtureof styrene, a lower styrene homolog, such as a-methylstyrene and anaphthol by the use of a suitable polymerization catalyst.

To this is added the zinc oxide in typical pigment to resin ratios offrom 3.5 :1 to 6.0:1. This mixture is then dispersed in any of severalmeans to reduce agglomerates to less than 1p. average particle size.This can be accomplished by ball milling or by the use of dispersingequipment as is found throughout the paint industry. The dispersedslurry is sensitized by adding a small quantity of an alcoholic solutionof the desired mixture of organic dyes.

The slurry may be coated on to the paper base by various methods usingeither a two-step application by an oxide or a one-step application bywire wound bar or gravure roller. After drying the coating is ready foruse in the conventional xerographic or electrophotographic process.

As previously noted, the dye blend which we use absorbs for the primarycolors and therefor has a magenta component, a cyan component and ayellow component. It has been ascertained that the magenta component isnicely supplanted by Rose Bengal either alone or in ad mixture withMerthiolate and Eosin Y. A better balance results with the mixture thanwith the Rose Bengal per se.

We use Merthiolate which is sodium ethyl mercury thiosalicylate as acatalyst. This compound is colorless but is dyed magenta by Eosin Y.

A yellow component for the absorption may be added to the dye blend andfor this purpose we may use the sodium salt of Fluorescein. Howeverabsorption for the blue and red may be obtained by a mixture ofChlorophyll and Patent Blue V with or without the Fluorescein.Furthermore, it is possible to supplant the Patent Blue V by Patent BlueA sold as Alphazurine A by National Division, Biological StainsDepartment, 40 Rector Street, New York. It is, of course, understoodthat inmodifying the blends by substituting one dye for another anadjustment in concentration must be provided for.

The dye components discussed above provide the necessary sensitizationand as a matter of fact when using Patent Blue A for Patent Blue V andby varying the concentration of the Patent Blue A, provision is made foravoiding the tendency of the background to yellow. It is preferredhowever to effect this result by incorporation of a separate maskingcomponent such as a blue pigment or a mixture of a coupling componentand a sensitized diazoamino compound which forms a blue dye on coupling.Such materials are available on the open market. Suitable materialscomprise a mixture of a Naphthol AS coupling component and a stabilizeddiazoamino compound of a 2, S-dialkoxy-4-benzoylamino aniline.Preferably, we use Naphthol ASBG in admixture with the diazos from2,5-dimethoxy-4-benzoylamino aniline and 2,5-diethoxy-4-benzoylaminoaniline, the azo groups being stabilized by N- methyl taurine. Thismixture will hereinafter be referred to as Blue Masking Dye Component.Another method for reducing the visible yellowing of background is toadd an inert light-fast blue dye not in the alcoholic sensitizingsolution but dissolved in a solvent common to the resin, such as tolueneor xylene. For this purpose we prefer to use a dye such as Heliogen BlueOil Soluble, a phthalocyanine type available from Antara Chemicals, NewYork. This solution will hereinafter be referred to as Blue Masking DyeComponent 2.

As is understandable, the ratios of the various components in thesensitizing composition will not vary to any substantial degree. We havefound that by utilizing the component in the following proportion perpound of zinc oxide that a recording paper meeting the full requirementsof the trade can be produced:

The following examples will serve to illustrate the invention but it isunderstood that the invention is not limited thereto.

EXAMPLE I A toluene solution was prepared of the following components:

vGrs. Epoxy ester resin (50%) 788 Velsicol TPO #1 78.8 Cobalt Napthenate(6%) -1 1.63 Manganese Naphthenate (6%) 1.63

'Into this was slurried 2100 grs. French Process zinc oxide and themixture dispersed on a standard pebble mill.

The slurry was then sensitized with a solution of the followingcomposition:

Rose Bengal mgs 176.4 Chlorophyll mgs 168.0 Patent Blue V mgs 27.3Fluorescein (Na) mgs 26.7 Cobalt acetate grs 1.78 Methanol ..cc 200 5.The sensitized dispersion was then coated on a clay coated sheet byconventional wire-wound rod technique. The coating thicknesses variedfrom 0.4 to 0.7 mil dry film thickness.

The product after drying was sensitive to the visible spectrum, gave agray appearance by reflected light, had high speed and excellentbalance. The usual loss of memory encountered in pre-illumination of thezinc oxide coatings was absent.

EXAMPLE II The procedure was the same as in Example I except that thefollowing components were employed:

The product obtained was superior in color balance to that of Example I.The background moreover was noticeably free from yellow stain.

EXAMPLE III The procedure was the same as in Example I excepting thatthe following sensitizing solution was employed, the amounts being interms of mg. per lb. of zinc oxide:

Merthiolate mg 4.79 Eosin Y mg 9.25 Rose Bengal mg 23.1 Chlorophyll mg22.1 Patent Blue A mg 12.1 Methanol on 8.33

EXAMPLE IV The procedure was the same as in Example II except that atoluene solution of 2.7 mgs. Heliogen Blue Oil Soluble per 100 grs. zincoxide, was added in place of Blue Masking Dye Component 1.

EXAMPLE V A mixture containing the followin ingredients was prepared:

Toluene l 1.25 Xylene ml 30 Epidene E168/50% solids grs 702 Cobaltnaphthenate (6%) grs 1.45 Manganese naphthenate (6%) grs 1.45 VelsioolTPO #1 grs 157 Zinc oxide grs 2100 This mixture was dispersed in a ballmill and then sensitized with Methanol cc 500 Merthiolate mgs 20 Eosin Ym2s 40 Rose Bengal mas 97 Chlorophyll mgs 93 Patent Blue V mgs.... 30Navy Blue mgs 27 The sensitized dispersion was then coated on paper,dried, exposed and developed with a liquid toner. Excellent results wereobtained.

Modifications of the invention will occur to persons skilled in the art.We therefore do not intend to be limited in the patent granted except asnecessitated by the appended claims.

We claim:

1. A storage stable electrophotographic recording element havingadequate speed and balance for whitelight exposure comprising a backingmember coated with finely divided photoconductive zinc oxide dispersedin an insulating resin binder, said zinc oxide being sensitized to lightrays corresponding to the primary colors by a blend of organic dyes inwhich the component absorptive of the green rays is selected from theclass consisting of Rose Bengal and mixtures thereof with Merthiolateand Eosin Y and the components absorptive of the red and blue rays areselected from the class consisting of (1) a mixture of Chlorophyll andPatent Blue V, (2) a mixture of Chlorophyll and Patent Blue A, and (3)mixtures of (1) and (2) with Fluorescein wherein the concentration ofthe components is adjusted so that when viewed by reflected light thesensitized zinc oxide appears white to neutral gray in color.

2. The article as defined in claim 1 wherein the blend of organic dyesincludes a blue dye for masking a yellow background, said blue dye beingformed in situ from a particulate mixture of a Naphthol AS couplingcomponent and a diazo compound obtained from a 2,5-di-alkoxy-4-benzoylamino aniline, the azo group of which is stabilized byN-methyltaurine.

3. The article as defined in claim 2 wherein the coupling component isNaphthol ASBO and the 2,5-di-alkoxy-4- benzoylamino aniline is a mixtureof the 2,5-dimethoxy and 2,5-diethoxy derivative.

4. The article as defined in claim 1 wherein in addition to the blend oforganic dyes is included in an oil soluble phthalocyanine dye for themasking of a yellow background.

5. The article as defined in claim 1 wherein the insulating resin-binderis an epoxy ester resin.

6. The article as defined in claim 1 wherein the insulating resin-binderis modified by a terpolymer derived from styrene, a lower styrenehomolog and a naphthol.

7. A storage stable electrophotographic recording element havingadequate speed and balance for white light exposure comprising a backingmember coated with finely divided photoconductive zinc oxide dispersedin an insulating resin binder, said zinc oxide :having adsorbed to thesurface thereof and being sensitized by a composition consisting of RoseBengal, Chlorophyll, Patent Blue V and Fluorescein wherein theconcentration of the components is adjusted so that when viewed byreflected light the sensitized zinc oxide appears white to neutral grayin color.

8. A storage stable electrophotographic recording ele-' ment havingadequate speed and balance for white light exposure comprising a backingmember coated with finely divided photoconductive zinc oxide dispersedin an insulating resin binder, said zinc oxide having adsorbed to thesurface thereof and being sensitized by a composition consisting of RoseBengal, Merthiolate, Eosin Y, Chlorophyll, Patent Blue V and the bluedye formed from a particulate mixture of a Naphthol AS couplingcomponent, the diazo of a 2,5-di-alkoxy-3-benzoylamino aniline, the azogroup of which is stabilized by N-methyl taurine wherein theconcentration of the components is adjusted so that when viewed byreflected light the sensitized zinc oxide appears white to neutral grayin color.

9. A storage stable electrophotographic recording element as defined inclaim 8 wherein the blue dye Heliogen Blue is used to mask the yellowbackground of the paper.

10. A storage stable electrophotographic recording element havingadequate speed and balance for white light exposure comprising a backingmember coated with finely divided photoconductive zinc oxide dispersedin an insulating resin binder, said zinc oxide having adsorbed to thesurface thereof and being sensitized by a composition consisting ofMerthiolate, Eosin Y, Rose Bengal, Chlorophyll and Patent Blue A whereinthe concentration of the components is adjusted so that when viewed byreflected light the sensitized zinc oxide appears whiteto neutral grayin color.

11. A photoconductive composition comprising finely dividedphotoconductive zinc oxide having a spectral response to white lightsuspended in an electrically insulating resin binder and havingincorporated therein a blend of organic dyes in which the componentabsorptive of the green rays is selected from the class consisting ofRose Bengal and mixtures thereof with Merthiolate and Eosin Y and thecomponent absorptive of the red and blue rays are (1) a mixture ofChlorophyll and Patent Blue V, (2) a mixture of Chlorophyll and PatentBlue A, and (3) mixtures of (1) and (2) with Fluorescein wherein theconcentration of the components is adjustedso that when viewed byreflected light the sensitized zinc-oxide appears white to neutral grayin color.-

12. The composition as defined in claim 11 wherein component, and adiazo compound from a 2,5-di-a1koxy.--

4-benzoylamino aniline, the azo group of which is stabilized byN-methyltaurine.

References Cited UNITED STATES PATENTS 3,051,569 3,052,540 '9/1962 Greig'96-1.7 3,245,706 4/1966 Cassiers et al. ;96,-1.8

NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

8/1962 Sugarma n et a1. 96--1.7

